Generally, a color image receiving tube comprises a panel 11 on which a fluorescent film 12 is coated and a funnel 13 attached to the back of the panel 11 as shown in FIG. 1. Inside the image receiving tube, an electron gun assembly 8 emitting thermion and a shadow mask 14 distinguishing colors are installed. A deflection yoke 15 is mounted on the outer surface of the funnel 13 in order to deflect the electron beams.
The electron gun assembly 8 is comprised of a cathode electrode heated and operated by a heater, a controlling electrode, a screen electrode, a focus electrode, an anode electrode and a shield cup which are placed in series in front of the cathode electrode.
Each electrode is placed apart from each other at regular intervals and supported by bead glass in the shape of a pole or a plate. Each electrode has three electron beam passage holes.
Further, at least two electrostatic lenses are formed in the electron gun assembly; one is a pre-focus lens formed by the potential differences between the applied voltage of the screen electrode and the applied voltage of the focus electrode, and the other is a main lens formed by the potential differences between the applied voltage of the focus electrode and the applied voltage of the anode electrode. The pre-focus lens helps prevent the electron beam which emits toward the main lens from scattering, while the main lens converges the electron beam on the screen.
If the central axis of the electron beam does not impact upon the center axis of the main lens, a halo phenomena results at the beam spots on the screen and the resolution of the color image receiving tube diminishes.
FIG. 2 is a perspective view of the focus electrode used in the prior art for a electron gun assembly. In the electrode body 1, three electron beam through holes a, b and c are arranged parallel to each other at regular intervals. If both external electron beams converge on the screen under each of the electron beam through holes of the focus electrode and the anode electrodes are not in accord with each other, an aberrations occur due to the distortion of the main lens, on the electron gun assembly of the prior art when both external electron beams are passing through the main lens, thereby creating a focusing characteristic of low quality.
An object of this invention is to provide an electron gun assembly that prevents the lowering of the focusing characteristic of a color image receiving tube caused by the aberration of the electron beam which is in turn caused by deviation of the center of electron beam through holes of the main lens and by uneven magnetic field phenomena such as a deflection yoke.